Merge pull request #3749 from maartenbreddels/fix_ipykernel_dep_8.x

fix: revert ipykernel dependency removal

Author: martinRenou

python/ipywidgets/setup.cfg

@@ -34,6 +34,7 @@ zip_safe = False
 packages = find:
 
 install_requires =
+  ipykernel>=4.5.1
   ipython>=6.1.0
   traitlets>=4.3.1
   widgetsnbextension~=4.0